In an automated information storage and retrieval system, also known as a library, numerous cells arrayed within the library are used to hold information media, such as magnetic tape cartridges or optical disks. (The term "cartridge" used herein refers to any retaining structure for information media. Although the present invention is described herein with respect to a magnetic tape library storing tape cartridges, it is not so limited but is equally applicable, for example, to an optical library holding optical disk cartridges, cartridge-less optical disks, optical tape cartridges and the like.) An accessor, furnished with a holding or gripping device and under the direction of a library controller, transports a selected cartridge between the cartridge's cell and a drive unit. The library controller is interconnected with a host device, such as a mainframe computer, which issues control signals related to reading/writing data from/to selected cartridges.
Library units also typically include an input/output station or port through which a system operator can pass a cartridge to be added to the array and through which the accessor can pass a cartridge to the operator for removal.
It will be appreciated that the library controller must "know" the identity of each cartridge and its location within the array of cells in order to be able to retrieve the cartridge for access. Typically, each cartridge has a unique identifying mark, such as a bar code label, on an edge of the cartridge visible through the cell opening. A vision system, such as a bar code reader, on the accessor can read the label when the accessor is positioned proximate to the cell. During an inventory of the library, such as when a new library has been installed, the vision system scans the cells and reports to the library controller the identity and location of stored cartridges. The library controller records the identity and location information in a database and later refers to it when a cartridge is to be retrieved or replaced or when a cartridge is imported from the outside and has to be stored in an empty cell. An inventory must also be taken each time a library access door is opened by the operator since it is not known whether the operator has added cartridges to the array, has removed cartridges, or has done nothing at all. If a cartridge label cannot be detected or read when the accessor is proximate to a cell, that cell is assumed to be empty.
However, the vision system may mistakenly classify a cell as being empty when, in fact, the cell is actually occupied. For example, a cartridge may occupy the cell but have an unreadable label, or even no label. A cartridge may be mis-oriented within the cell or a foreign object may occupy the cell. If the accessor attempts to insert a cartridge into such an occupied cell, an error condition will result causing a time consuming error recovery procedure to be initiated.
One known system attempts to reduce such non-empty errors by directing the accessor's gripper to attempt to reach inside every cell which does not have an identifiable cartridge and verify by "touch" that the cell is actually empty. Any cell which is still not found to be empty is classified in the database as having an "occupied but invalid" status to prevent its use. An inventory of a 400 cell library, for example, can take three or four hours to complete using this system.